This grant proposes to study how embryonic motor neuron axons are guided to their appropriate muscle targets. These cells directly mediate the nervous system's control of respiration and movement. Thus, diseases of motor neurons such as ALS and SMA have devastating consequences for human health and care. Our studies should provide insight into how these cells develop and over the long term contribute to our general understanding of the mechanisms that control neuronal connectivity and circuit formation in the brain. During development, motor neuron subtypes are generated that exhibit distinct cell migration patterns and specific preferences for axon pathways. Although families of transcription factors have been identified in motor neuron subtypes, less is known about the molecular signals that control the connectivity of individual motor neuron subtypes in mammals. Our preliminary studies have implicated a family of receptor tyrosine kinases in axonal navigation, which others have found are proto-oncogenes in non-neuronal tissues. In aim 1 we will characterize how EphA and ephrin-A signaling is used to guide both MMCm and LMCI cells. These studies will help to understand how axon guidance molecules expand their repertoire of functions. In aim 2 we will examine inter-axonal interactions between motor and sensory neurons to understand how proper afferent and efferent pathways develop, focusing on the role of EphAs and ephrin-As present on motor and sensory neuron axons. In aim 3 we will characterize the role of FgfR1 in MMCm motor neuron axon guidance and study FgfR1-EphA4 receptor "cross-talk". This will help to understand how attractive and repulsive guidance cues are integrated by MMCm growth cones. In aim 4 we will identify coreceptors needed for reverse signaling by the ephrin-A GPI-anchored proteins and determine whether other GPI-anchored proteins function as motor axon guidance molecules.